Wheels for the back of an A Body barracuda....?

One easy trick that you can do to put your mind at ease:
Mount up some stock wheels and tires and get under the car with a tape measure. Look around and see how much clearance there is between the tire and leaf springs and between the tire and outer wheel edge at the quarter panel. Chances are with stock wheels and tires, there will be a LOT of clearance. Then you can theoretically "add" width to arrive at what will fit.
Example:
A 14x4 wheel that has 3 inches of clearance on the inside to the leaf springs could be 2 inches wider and still clear by a comfortable margin. Same principle goes for the clearance to the outside. Add these numbers to an existing wheel and you get the idea of what fits.
I have fit 15x7 wheels in an early A body with some room to spare. A 15x8 fits but you have to be really careful about the tire. The diameter cannot exceed 25-26 inches or the tire wants to contact the bottom edge of the wheel opening at the front. This limits you to a 235-60-15 as far as height.